1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device capable of digital driving, and a method for manufacturing the same.
2. Discussion of the Related Art
Generally, an organic electroluminescent device (referred to hereinafter simply as an “organic EL device”) is operated in such a manner that, when electric charges are injected into an organic film formed between an electron injection electrode (anode) and a hole injection electrode (cathode), electrons and holes are recombined and then extinguished, thereby generating light. Such an organic EL device has been evaluated as a next generation display device having characteristics of low driving voltage and low power consumption.
Construction of a conventional organic EL device and a method for manufacturing the same will be described with reference to the accompanying drawings.
FIG. 1 illustrates a conventional organic EL device.
As shown in FIG. 1, the organic EL device has an anode 102 formed on a transparent substrate 101. A material for the anode 102 generally comprises indium tin oxide (ITO), and the anode 102 is surface-treated using O2 plasma, UVO, and the like after the substrate 101 is coated with ITO. When impurities on the surface of the anode 102 are removed by surface treatment as described above, interface properties between the anode and a hole injection layer are enhanced, thereby allowing easy injection of holes.
Then, the hole injection layer (which will also be referred to “HIL”) 103 is formed on the anode 102. As for the HIL 103, copper phthalocyanine (CuPc) is generally coated to a thickness of about 10˜30 nm on the anode 102.
A hole transport layer (HTL) 104 is formed on the HIL 103. As the HLT 104, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′biphenylyl)-4,4′-diamine (TPD) or 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPD) is formed to a thickness of about 30˜60 nm on the HIL 103.
An organic emitting layer 105 is formed on the HTL 104. At this time, dopants can be added, if necessary. In the case of a green light emitting device, tris(8-hydroxy-quinolate)aluminum (Alq3) as the organic emitting layer 105 is typically deposited to a thickness of about 30˜60 nm, and coumine derivative (C545T) or quinacridone (Qd) so the dopant is used. In the case of a red light emitting device, Alq3 is used as the organic emitting layer 105, and DCM, DCJT, DCJTB or the like is used as the dopant. In the case of a blue light emitting device, DPVBi is generally used as the organic emitting layer 105 without any dopant.
An electron transport layer (which will also be referred to as “ETL”) 106, and an electron injecting layer (which will also be referred to as “EIL”) 107 are sequentially formed on the organic emitting layer 105. In the case of the green light emitting device, since Alq3 has good electron transportation capabilities, it is not necessary to form the ETL 106 and the EIL 107 thereon.
For the electron injecting layer 107, LiF or Li2O is thinly deposited to a thickness of about 5 Å, otherwise alkali metal or alkaline earth metal such as Li, Ca, Mg, Sm and the like is deposited to a thickness of about 200 □, thereby allowing easy injection of electrons.
For a cathode 108, Al is coated to a thickness of about 1,000 Å on the electron injecting layer 107, and a closing plate (not shown) containing a hygroscopic agent is bonded to the cathode 108 using an ultraviolet curable bonding agent thereby protecting the organic EL device from moisture or O2 in the atmosphere.
The organic EL device constructed as described above may suffer great changes in life span and efficiency according to materials, surface treatment conditions for the anode, and stacked structures of the organic EL device.
Generally, it is important for the organic EL device to have an extended life span and a stabilized interface between the layers for stable current injection.
However, in the organic EL device, the interfaces between inorganic layers and organic layers cause a deterioration of the device.
In the organic EL device, the interfaces between the inorganic layers and the organic layers include an interface between the anode and the hole injection layer, and an interface between the electron transport layer and the electron injecting layer (or the cathode). Especially, the interface between the anode and the hole injection layer is the most influential upon deterioration of the device.
Thus, according to the prior art, in order to solve this problem, the HIL is deposited on the anode after the anode formed of ITO is surface-treated using O2 plasma or UVO such that the impurities are removed from the surface of the anode 102.
As a result, the anode is enhanced in adhesion to the hole injection layer, so that life span of the device is extended, and current is stably injected therefrom.
However, with such a method, there is a limitation in extending the life span of the device.
FIG. 2 is a graph depicting results of a test of constant current acceleration for a green light emitting device having a surface treated anode, and FIG. 3 is a graph depicting results of a test of constant voltage acceleration for the green light emitting device having the surface treated anode.
As shown in FIG. 2, when the green light emitting device is driven in a constant current mode, a voltage applied to the device is increased. As a result, the device suffers reduction in brightness of 50% compared to an initial brightness. For the green light emitting device, the voltage is increased by approximately 2 V.
Additionally, as shown in FIG. 3, when the green light emitting device is driven in a constant voltage mode, current applied to the device is increased along with deterioration of the device. As a result, the brightness of the green light emitting device is sharply decreased 10 times of less of the brightness of the case where the device is driven in the constant current mode.
As such, in the organic EL device, the voltage increase upon constant current driving and the current decrease upon constant voltage driving are mostly caused by deterioration in the interfaces between the organic materials and the inorganic materials rather than deterioration of the organic material itself. In particular, among the interfaces between the organic materials and the inorganic materials, the interface between the anode formed of the organic material and the hole injection layer is the most influential upon deterioration of the device.